This invention relates generally to traveling water screens for filtering water being supplied from a reservoir through a channel or sluice to a hydroelectric, milling, or similar water abstraction facility, and particularly to a dual-flow band screen conversion apparatus for replacing a through-flow band screen device without modification of the structural foundation of the channel.
In addition to the body of information disclosed in the published patent documents of various countries regarding the subject matter identified above, an adequate technical overview of through-flow and dual-flow band screen filtration devices may be found in the recently published text Screening Equipment Handbook: For Industrial and Municipal Water and Wastewater Treatment by Tom M. Pankratz (Technomic Publishing Company, Lancaster, Penna. 1988)(LC 88-50738).
U.S. Pat. No. 1,208,655 to Reynolds represents one of the first references disclosing a traveling water flume-screen of the through-flow type, wherein separate articulated screen sections each have marginal frames secured to endless chains carried over sprocketed wheels so that the screens move through a circuitous, predominantly vertical loop. The traveling screens form two imaginary planes or generatrices which are disposed generally perpendicular to the flow of water through the channel. In the Reynolds '655 device, the water is redirected vertically upward after passing through an opening in the structural foundation between a channel and a chamber surrounding the filtration apparatus, into the forward generatrix at an angle less than perpendicular, downwardly behind the rear generatrix, and through a rear intake conduit
As the screens are carried above the surface level of the water, debris collected on the screens may be washed off the screens and into a gutter with a high pressure jet created by spray nozzles located within the interior of the band screen apparatus. A small percentage of that debris may remain on the screens as they pass downwardly along the rear generatrix, and ultimately be dislodged by the higher water pressure created by the normal flow through the submerged screens. Even small quantities of such carry-over debris may prove deleterious to the pumps and water handling equipment within the water abstraction facility.
U.S. Pat. No. 2,996,189 to Salterbach discloses a plurality of normally stationary or immobile through-flow filtration screen assemblies which are slidably received within guide tracks or grooves in the side walls of the structural foundation, the screen assemblies alternately being lifted from the channel to be completely cleaned, either automatically or manually.
U.S. Pat. No. 3,850,804 to Taylor combines the teachings of the previous references to achieve a traveling water screen device with an endless series or band of screens which may be received within metal guides set in the side walls or columns of the foundation of a completely open channel or sluice. The Taylor '804 device permits more convenient installation, repair, and replacement of the entire band screen filtering apparatus.
A diagrammatic representation of one possible through-flow band screen apparatus is shown in FIG. 2, with the general flow of water shown by directional arrows. It may also be readily appreciated that in addition to the problem of carry-over debris, through-flow band screen devices are subject to other drawbacks. In a deep channel with a substantially unobstructed flow of water, the pressure exerted directly on the screens of a through-flow device will require significant reinforcement of the screens and frame, cause damage to those screens, or result in debris being imbedded in the screens. Having the water pass through two screens additionally creates unnecessary static pressure without enhancing the filtration capabilities of the device.
To eliminate some of the problems associated with through-flow screening devices, dual-flow traveling water screen were developed that require the moving water to flow only through a single screen element while traversing the channel, with any one screen element having water passing through that screen in only one direction over any given period. This may be contrasted with through-flow devices where water passes in one direction through a screen along the front generatrix, and in the reverse direction as the screen travels along the rear generatrix.
Dual-flow travelling water screens may be one of two types:
(a) Double entry/single exit devices where the water passes around the exterior of the band screen apparatus, inwardly through the screens from each opposing side of the apparatus, and outwardly through an exit conduit positioned between the generatrices and communicating with the interior of the band screen apparatus, or
(b) Single entry/double exit devices where the water flows through a port between the generatrices and into the interior of the band screen apparatus, exiting through either of the opposing screens. A double entry/single exit band screen conversion device is represented diagrammatically in FIG. 3 with the flow of water shown by arrows. A single entry/double exit device could be represented by the same FIG. 3, but with the direction of the arrows showing the flow of water being reversed from that shown in FIG. 3.
One representative example of early dual-flow band screen devices may be seen in the French Brevet d'Invention No. 577,255 which discloses both types of entry/exit systems, the double entry configuration having angled foundation side walls in the area of the channel proximate to the band screen device, and the single entry configuration appearing to have support for the band screen apparatus from the foundation both at the front and rear corners of the band screen apparatus.
U.S. Pat. No. 1,726,608 to Brackett discloses a self-contained rotary strainer which utilizes dual-flow principles in a more unusual single entry/single exit configuration, with a restricted side outlet conduit. The Brackett '608 device is usable only at low flow volumes, or with high static pressure and a substantially water-tight seal between the housing and screen frames.
Double entry dual-flow configurations are generally preferred because they create less static back pressure, however it can be seen from the French '255 patent that the double entry configurations generally require significant structural reinforcement of the band screen main frame or housing to withstand the upstream flow pressure exerted against the submerged front main frame or housing wall, or alternately require that the foundation be specifically constructed to receive the particular band screen apparatus.
While dual-flow band screen devices are more common in European countries, with modern sluices and filtration channels being designed to utilize that technology, the majority of similar facilities in the United States have been or are continuing to be designed to use through-flow technology. Consequently, the ability to convert existing through-flow sluices and channels to dual-flow technology is a concern
One dual-flow band screen apparatus, and a method for substituting that apparatus for its through-flow counterpart in existing through-flow channels, is shown in U.S. Pat. No. 4,374,736 to Jackson. The Jackson '736 device comprises a two-part assembly including a rear support wall and a band screen apparatus, such as is shown diagrammatically in FIG. 3 herein. In operation, the channel is dammed and drained, and the rear support wall R is lowered into place and normally installed against a rear foundation wall. The rear support wall engages the guide tracks defined in the side walls of the foundation in which the housing of the through-flow apparatus is normally received, and extends rearwardly parallel to the side walls and then inwardly perpendicular to the side walls. The rear support wall defines an exit port communicating with the outlet or opening in the rear foundation wall against which the rear support wall is fixedly mounted, the majority of the rearward pressure or forces being absorbed by the rear foundation wall rather than the rear support wall. The exit port similarly communicates with the interior of the band screen apparatus between the generatrices once the band screen apparatus is installed and mounted to the rear support wall.
While the Jackson '736 patent does present one solution for situations where a conventional through-flow apparatus is being replaced by a dual-flow apparatus and it is desirable to utilize the existing foundation structure with minimal modification, there are several drawbacks inherent in its design. The Jackson '736 device requires significant expenditures for installation, because the water abstraction facility must be shut down, the channel dammed and drained, and the rear support wall installed prior to installing the band screen apparatus. Once installed, if the band screen device needs repair or replacement the entire operation must be repeated, resulting in additional time and expense.
The Jackson '736 device also requires significant structural reinforcement of the band screen apparatus itself, since the submerged portion of the front wall must withstand the pressure of the upstream flow through the channel. The structural reinforcement must also be capable of preventing lateral or sideways movement or shifting within the channel, particularly since the water flowing around the front housing wall and rebounding against the rear support wall creates swirling turbulence which can generate destructive vibrations in the band screen apparatus. This is a particular concern in deeper channels where the front wall of the band screen apparatus is exposed, since significant water pressure differentials can be created between the surface and the bottom of the channel, and vertical turbulence can result in tremendous lateral pressure on the band screen apparatus. Consequently, a significant amount of cross-bracing and lattice work must be incorporated into the Jackson '736 design while avoiding interference with the operation of the band screen apparatus.
The French Brevet d'Invention No. 2,110,809 discloses a dual-flow water filtering system in which a band screen apparatus is mounted within a generally cylindrical housing having a restricted front inlet and a rear outlet, thereby creating a self-contained, freestanding unit for smaller volume filtering applications where turbulence and pressures are minimal.